Japanese Patent No. 4067021 describes a motor controller. The motor controller includes a rectifying unit and an inverter. The rectifying unit and the inverter are mutually connected through a DC link. The rectifying unit receives an AC voltage, converts the AC voltage into a DC voltage using a full-wave rectification, and outputs the DC voltage to the DC link. The inverter receives the DC voltage, converts the DC voltage into an AC voltage, and outputs the AC voltage to a motor.
The DC link includes an LC filter including a reactor and a capacitor. More specifically, the capacitor and the reactor are connected in series with each other between a pair of output ends of the rectifying unit. Then, a voltage across the capacitor is input to the inverter as a DC voltage. The capacitance of the capacitor is smaller than those of generally-called smoothing capacitors. The voltage across the capacitor has a ripple component caused by the full-wave rectification. As such, when the capacitance of the capacitor included in the DC link is smaller, the DC link as well as the rectifying unit and the inverter that are coupled through the DC link are sometimes collectively referred to as a “capacitor-less inverter”.
In Japanese Patent No. 4067021, the inverter is controlled on the basis of a voltage across the reactor to reduce a harmonic component, which is caused by the resonance of the LC filter, of a DC voltage. For example, an initial value of a voltage control ratio of the inverter is corrected by subtracting from the initial value a product of the voltage across the reactor and a gain to calculate a target value of the voltage control ratio. Then, a control signal for the inverter is generated on the basis of this target value of the voltage control ratio and a voltage command value calculated using a known method. Accordingly, the harmonic component of the voltage across the capacitor is reduced, and furthermore, distortions of the current input to the motor controller are reduced. Such a control based on the voltage of the reactor is also referred to as a VL control system in the present application.
Japanese Patent No. 4750553 proposes a technique for suppressing the resonance of a DC voltage to be input to an inverter on the basis of a voltage across a capacitor.